A. Technical Field
The present invention relates to a production process for an unsaturated aldehyde and/or an unsaturated carboxylic acid. More particularly, the present invention relates to a process comprising the step of carrying out catalytic gas phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether as a raw material with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with catalysts, thereby producing an unsaturated aldehyde and/or an unsaturated carboxylic acid.
B. Background Art
As to a process which comprises the step of carrying out catalytic gas phase oxidation of at least one compound selected from the group consisting of propylene, isobutylene, t-butyl alcohol, and methyl t-butyl ether as a raw material with molecular oxygen or a molecular-oxygen-containing gas by using a fixed-bed multitubular reactor which is packed with a catalyst, thereby producing an unsaturated aldehyde and/or an unsaturated carboxylic acid that correspond to each raw material, several proposals have hitherto been reported (e.g. JP-B-030688/1978, JP-B-038331/1988, JP-A-294238/1991, JP-A-294239/1991, JP-A-217932/1992, JP-A-003093/1996, and JP-A-168003/1998). Of the above, some processes are industrially carried out.
This catalytic gas phase oxidation reaction is accompanied with an extremely exothermic reaction, and therefore a local portion having an extraordinarily high temperature (which may hereinafter be referred to as a hot spot portion) occurs in a catalyst layer. Particularly, as long as the oxidation reaction is carried out by using the fixed-bed multitubular reactor, it is impossible to avoid the occurrence of the hot spot portion in the catalyst layer.
When the hot spot portion has a high temperature, it may excessively cause the oxidation reaction, resulting in a low yield, or, in the worst case, it may cause a runaway reaction. In addition, a catalyst as located at the hot spot portion is exposed to the high temperature, and therefore there is accelerated the deterioration of the catalyst, such as changes of physical properties and chemical properties of the catalyst to result in lowering its activity and the selectivity of the objective product. Particularly, in the case of a molybdenum-containing catalyst, the composition and properties of the catalyst tend to change due to sublimation of the molybdenum component, and therefore the deterioration extent of the catalyst is large.
The above problems are more striking in the case of carrying out the reaction at a high space velocity or in a high concentration of the raw gas for the purpose of enhancing the productivity of the objective product.
As to the above problems, if attention is directed to the entirety of the catalyst layer as packed in the reaction tube, then the catalyst as located at the hot spot portion is more rapidly deteriorated than a catalyst as located at the other portions, and the yield of the objective product is greatly lowered due to longtime use, so its production can be difficult to stably carry out. And then, as is aforementioned, the deterioration extent of the catalyst is particularly large in the case of the molybdenum-containing catalyst or in the case of carrying out the reaction at a high space velocity or in a high concentration of the raw gas.
Any of the aforementioned known-in-public proposals having hitherto been made is a proposal that aims at suppressing the temperature of the hot spot portion to low. However, in the case of carrying out the oxidation reaction by using the fixed-bed multitubular reactor, these proposals cannot completely prevent the occurrence of the hot spot portion in the catalyst layer and therefore have not succeeded in solving the problem such that the deterioration extent of the catalyst as located at the hot spot portion is relatively larger than that of a catalyst as located at the other portions. This problem is remarkable particularly in the case of using the molybdenum-containing catalyst or in the case of carrying out the reaction in a high concentration of the raw gas.